Remediation of large-scale model experiment for heavy metal contaminated clayey soil: An optimized mass transfer process strategy of freeze-thaw and washing

Abstract

To reduce the limitations of the chemical washing method due to soil texture, the freezing technique has been used to enhance the mass transfer efficiency between the eluent and heavy metal contaminants. With the optimized water supplement and cooling conditions, a large-scale model experiment was conducted. Results show that the optimized method is prone to enhancing the eluent intake of clayey soil, leading to an increase in the amount of soil frost heave, which has a feedback effect by breaking the soil structure and enhancing the eluent intake. The synergistic effect of the two is responsible for the improvement of washing efficiency. In addition, after four freeze-thaw and washing cycles, the total removal rates of the metallic elements Pb and Cd reach 58.73% and 69.04%, respectively, satisfying the soil environmental quality standard. The soil bioavailability is effectively reduced by alternating the eluent with ethylenediaminetetraacetic acid (EDTA) and water, thereby reducing the bioavailability of Pb and Cd by 66.01% and 74.18%, respectively. This remediation strategy for heavy metals in clayey soils utilizes alternate freezing and thawing (natural cold energy) in seasonally frozen regions, which is beneficial for constructing an ecological civilization and protecting the environment.